1. Field of the Invention
This invention relates to molded case circuit breakers and more particularly to a cam roll pin assembly having independent rollers for engaging the different contact surfaces to reduce frictional and torsional forces on the cam roll pin during operation.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit. An overload condition is about 200-300% of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000% or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation the contacts may be opened by an operating mechanism or by a magnetic repulsion member. The magnetic repulsion member causes the contacts to separate under relatively high level short circuit conditions. More particularly, the magnetic repulsion member is connected between a pivotally mounted contact arm and a stationary conductor. The magnetic repulsion member is a generally V-shaped member defining two legs. During high level short circuit conditions, magnetic repulsion forces are generated between the legs of the magnetic repulsion member as a result of the current flowing therethrough which, in turn, causes the pivotally mounted contact arm to open.
In a multipole circuit breaker, such as a three-pole circuit breaker, three separate contact assemblies having magnetic repulsion members are provided; one for each pole. The contact arm assemblies are operated independently by the magnetic repulsion members. For example, for a high level short circuit on the A phase, only the A phase contacts would be blown open by its respective magnetic repulsion member. The magnetic repulsion members for the B and C phases would be unaffected by the operation of the A phase contact assembly. The circuit breaker operating mechanism is used to trip the other two poles in such a situation. This is done to prevent a condition known as single phasing, which can occur for circuit breakers connected to rotational loads, such as motors. In such a situation, unless all phases are tripped, the motor may act as a generator and feed the fault.
In the other automatic mode of operation, the contact assemblies for all three poles are tripped together by a current sensing circuit and a mechanical operating mechanism. More particularly, current transformers are provided within the circuit breaker housing to sense overcurrent conditions. When an overcurrent condition is sensed, the current transformers provide a signal to electronic circuitry which actuates the operating mechanism to cause the contacts to be separated.
A cam roll pin assembly is provided to maintain contact force between the movable and stationary contacts. The cam roll pin engages a cam surface disposed at the rear of the contact assembly to cause the contact arms to pivot about a first pivot point in the contact assembly. The ends of the cam roll pin also engage a slot which carries the cam roll pin.
During overcurrent conditions less than the withstand rating of the circuit breaker, the cam roll pin maintains the contact force between the movable and fixed contacts to allow the operating mechanism to open the contacts. Above the withstand rating of the circuit breaker, the cam roll pin rotates away from the cam surface to allow the contact assembly to blow open. This condition results in the contact arms pivoting about a second pivot point.
During a blow open condition the contact points on the cam roll pin rotate in different directions. Conventionally, one piece cam roll pins are utilized. Under such conditions, the one piece cam roll pin is subject to relatively large functional forces which can reduce the blow open speed. Moreover, such conditions may cause the breaker not to trip the remaining poles after a blow open condition has occurred.